The present invention relates to a technology for maintaining and repairing a structure in an underwater, and for example, to an apparatus for preventably maintaining and repairing a reactor pressure vessel and an in-core or in-pile (herein, which may be referred to as xe2x80x9cin-corexe2x80x9d for the sake of convenience) structure in a nuclear power (generation) plant or the like. In particular, the present invention relates to underwater maintenance and repair apparatus and method, which can achieve stress improvement of surface layer in the vicinity of a weld line (seam), surface modification of sensitized metal fiber and achieve welding repair with respect to the following targets under the underwater environment such as cooling water of reactor pressure vessel. In this case, the stress improvement is carried out so as to change a residual tensile stress generated by heat influence in welding performance into a compression stress by a laser. Further, the above targets include a space partitioned by a shroud shell outer wall, which is an in-core structure, a baffle plate and a reactor pressure vessel inner wall, and a welded structure surface existing in a space surrounded by other in-core structures.
Conventionally, an in-core structure of a light water reactor, for example, a boiled water reactor has been composed of a material having sufficient corrosion resistance and high temperature strength under the high temperature and high pressure environment, for example, austenite stainless steel or nickel-group alloy.
However, a non-replaceable member of the in-core structure is exposed to a severe environment by a long-period operation of plant, and for this reason, the non-replaceable member receives an influence of neutron irradiation. As a result, a problem arises such that used materials are deteriorated. In particular, the vicinity of a welded portion of the in-core structure has a possibility of potential stress corrosion cracking due to material sensitization by welding input heat and the influence by tensile residual stress.
Recently, in order to stably and safely operate a nuclear power generation plant, a surface modification technology of various materials has been developed as preventive maintenance means. There is a technology of irradiating a laser beam to the surface of material so as to achieve surface modification. The technology has been disclosed in Japanese Patent Laid-Open Publications No. HEI 7-246483 and No. HEI 8-206869, for example.
In the above conventional examples, the technology described in the former is a laser peeling method, in which a laser beam emitted from a pulse laser device is irradiated to the surface (working surface) of a workpiece via a reflection mirror, and then, a residual tensile stress on the working surface is changed into a compression stress while varying the irradiating position on the working surface.
On the other hand, the technology described in the latter is an underwater laser machining method, in which a high output laser beam having a visible wavelength and a short pulse is irradiated to a working surface immersed in a cooling water, and thereby, a residual stress on the working surface is improved, thus eliminating crack or clad.
In the above conventional methods, an optical fiber cable is used to transmit a laser beam, and then, the laser beam is supplied to an in-core target via the optical fiber cable so as to work the working surface of the in-core structure.
In this case, the optical fiber cable used to transmit a laser beam considering a radiation resistance has a merit such that it is thin and light. However, the optical fiber cable has the following demerit. That is, the optical fiber cable has a large allowable bend radius (about 500 mm) and is easy to be broken when an external force is locally applied to the optical fiber cable (i.e., in a case where allowable or more bending, twist and tensile stress are applied to the optical fiber cable). For this reason, the optical fiber cable is moved to a place having a wide space such that the bend radius is allowable, without causing a problem, and the laser beam is irradiated to a working portion of the place via the optical fiber cable. However, in a complicate and narrow place, a problem arises such that it is impossible to close the optical fiber cable to the working portion due to contact with others.
Further, in the case of moving a working head attached with the optical fiber cable to a working position, during movement, an external force such as twist and tension is applied to the optical fiber cable, and for this reason, the optical fiber cable needs to be carefully handled. In the reactor pressure vessel having complicate and narrow portions, a problem arises such that it is excessive load for a worker to monitor a state that an external force is applied to the optical fiber cable and to take suitable measures thereto.
As described above, the optical fiber cable has a large bend radius and is easy to be broken. Therefore, the nuclear power generation plant needs to have construction and structure such that allowable or more bending, twist and tensile force are not applied to the optical fiber cable. Further, in the case of manufacturing an apparatus for the plant, the apparatus needs to have a structure such that the apparatus is quickly replaceable when it is broken down in error.
In the structure in the reactor pressure vessel, which is a target for preventive maintenance and repair of the reactor, a complicate and narrow portion exists. For example, as shown in FIG. 16, there exists a space (hereinafter, referred to as annulus space) surrounded by an inner wall of reactor pressure vessel 1, an outer wall of shroud 2 and a baffle plate 3. The space is very narrow because a jet pump 4 exists therein. Eight or more jet pumps 4 are arranged around the shroud 2. A welded structure such as jet pump 4 existing in the annulus portion has a complicate structure.
In the case of applying a preventive maintenance and repair apparatus for an underwater structure, in the use of laser, to the surface of the welded structure located in the annulus space, safety and reliability in handling of the optical fiber cable are very important.
In particular, in the welded structure of the annulus space, there exist a welding portion between a riser brass arm 6 fixing a riser pipe 5 of the jet pump 4 to the reactor pressure vessel 1 and the reactor pressure vessel 1, and a welding portion between the riser brass arm 6 and the riser pipe 5. These welding portions are positioned in a very complicate and narrow space, and for this reason, the apparatus must be made into a small size, and further, flexibility must be improved, in addition to the safe and reliable handling of the optical fiber cable.
The present invention has been made in view of the above circumstances. Therefore, an object of the present invention is to provide underwater maintenance and repair apparatus and method, which can achieve surface modification, surface machining and decontamination by precisely irradiating a laser beam in the underwater to a welding portion of an in-core structure located in a narrow space, such as a riser brass arm of a jet pump located in an annulus space of a reactor pressure vessel.
In order to achieve the above object, the present invention provides an underwater maintenance and repair apparatus, comprising: a laser oscillator for generating a laser beam; a working head movably attached to a portion in a vicinity of a working target portion in the underwater and adapted to scan the laser beam to the working target portion; an optical fiber cable for optically connecting the working head to the laser oscillator; and a joint mechanism arranged on a connective portion of the optical fiber cable and the working head to be separable through a remote control operation, wherein the working head irradiates the laser beam to a surface of the working target portion so as to achieve surface modification, surface working and decontamination.
Further, the present invention also provides an underwater maintenance and repair method, comprising the steps of: independently fixing a working head to a portion in a vicinity of a working target portion; jointing an optical fiber cable to the working head using a joint mechanism through a remote control operation; guiding a laser beam emitted from a laser oscillator via the optical fiber cable; and irradiating the laser beam from the working head to a surface of the working target portion while carrying out a scanning operation to the surface to achieve surface modification, surface working and decontamination.
According to the present invention, in the case of attaching the working head to a complicate and narrow portion, the optical fiber cable, which is lack of flexibility and breaks down when an external force exceeding an allowable range is locally applied, is removed from the working head. In this state, the working head is first attached to the working target portion, and thereafter, the optical fiber cable is attached to the working head. According to this structure, the working head, the cable and hose attached to the working head and the optical fiber cable are handled independently from each other, and thereby, it becomes possible to achieve surface modification, surface working and decontamination with respect to a complicate and narrow working target portion without breaking the optical fiber cable.
Preferred embodiments, modification examples and their operation and effect of the present invention will be further apparent from the following descriptions with reference to the accompanying drawings.
Incidentally, although the present invention is applicable to various liquid environments, it is one preferred embodiment to apply the present invention to maintenance and repair work in a reactor pressure vessel as in the present invention.